The present invention relates generally to a tubular implantable prosthesis formed of porous expanded polytetrafluoroethylene. More particularly, the present invention relates to a composite, multi-layered endoprosthesis having increased axial and radial compliance.
An intraluminal prosthesis is a medical device commonly known to be used in the treatment of diseased blood vessels. An intraluminal prosthesis is typically used to repair, replace, or otherwise correct a damaged blood vessel. An artery or vein may be diseased in a variety of different ways. The prosthesis may therefore be used to prevent or treat a wide variety of defects such as stenosis of the vessel, thrombosis, occlusion, or an aneurysm.
One type of endoluminal prosthesis used in the repair of diseases in various body vessels is a stent. A stent is a generally longitudinal tubular device formed of biocompatible material which is useful to open and support various lumens in the body. For example, stents may used in the vascular system, urogenital tract and bile duct, as well as in a variety of other applications in the body. Endovascular stents have become widely used for the treatment of stenosis, strictures, and aneurysms in various blood vessels. These devices are implanted within the vessel to open and/or reinforce collapsing or partially occluded sections of the vessel.
Stents are generally open ended and are radially expandable between a generally unexpended insertion diameter and an expanded implantation diameter which is greater than the unexpended insertion diameter. Stents are often flexible in configuration, which allows them to be inserted through and conform to tortuous pathways in the blood vessel. The stent is generally inserted in a radially compressed state and expanded either through a self-expanding mechanism, or through the use of balloon catheters.
A graft is another type of commonly known type of intraluminal prosthesis which is used to repair and replace various body vessels. A graft provides an artificial lumen through which blood may flow. Grafts are tubular devices which may be formed of a variety of material, including textiles, and non-textile materials. One type of non-textile material particularly useful as an implantable intraluminal prosthesis is polytetrafluoroethylene (PTFE). PTFE exhibits superior biocompatability and low thrombogenicity, which makes it particularly useful as vascular graft material in the repair or replacement of blood vessels. In vascular applications, the grafts are manufactured from expanded polytetrafluoroethylene (ePTFE) tubes. These tubes have a microporous structure which allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft. These tubes may be formed from extruded tubes or may be formed from a sheet of films formed into tubes.
Grafts formed of ePTFE have a fibrous state which is defined by interspaced nodes interconnected by elongated fibrils. The spaces between the node surfaces that is spanned by the fibrils is defined as the internodal distance (IND). Porosity of a graft is measured generally by IND. In order of proper tissue ingrowth and cell endothelization, grafts must have sufficient porosity obtained through expansion. When the term expanded is used to describe PTFE, it is intended to describe PTFE which has been stretched, in accordance with techniques which increase IND and concomitantly porosity. The stretching may be in uni-axially, bi-axially, or multi-axially. The nodes are spaced apart by the stretched fibrils in the direction of the expansion. Properties such as tensile strength, tear strength and radial (hoop) strength are all dependent on the expansion process. Expanding the film by stretching it in two directions that are substantially perpendicular to each other, for example longitudinally and transversely, creates a biaxially oriented material. Films having multi-axially-oriented fibrils may also be made by expanding the film in more than two directions. Porous ePTFE grafts have their greatest strength in directions parallel to the orientation of their fibrils. With the increased strength, however, often comes reduced flexibility.
While ePTFE has been described above as having desirable biocompatability qualities, tubes comprised of ePTFE, as well as films made into tubes, tend to exhibit axial stiffness, and minimal radial compliance. Longitudinal compliance is of particular importance to intraluminal prosthesis as the device must be delivered through tortuous pathways of a blood vessel to the implantation site where it is expanded. A reduction in axial and radial flexibility makes intraluminal delivery more difficult.
Composite intraluminal prosthesis are known in the art. In particular, it is known to combine a stent and a graft to form a composite medical device. Such composite medical devices provide additional support for blood flow through weakened sections of a blood vessel. In endovascular applications the use of a composite graft or a stent/graft combination is becoming increasingly important because the combination not only effectively allows the passage of blood therethrough, but also ensures patency of the implant. But, composite prosthesis, especially those consisting of ePTFE, while exhibiting superior biocompatability qualities, also exhibit decreased axial and radial compliance. It is therefore desirable to provide an ePTFE composite intraluminal prosthesis which exhibits increased axial and radial compliance.
The present invention comprises a composite ePTFE vascular prosthesis. The composite has three layers; an inner tubular ePTFE layer, a discontinuous outer layer, and a radially deformable stent atop the inner tubular layer and entirely beneath the outer layer.
One advantage of the present invention is that it provides an improved composite ePTFE intraluminal prosthesis exhibiting increased axial and circumferential compliance and flexibility and greater tissue ingrowth.
Another advantage of the present invention is that it provides an improved stent/graft combination, exhibiting increased axial and circumferential compliance and flexibility.
Another advantage of the present invention is that it provides an improved composite ePTFE intraluminal prosthesis exhibiting increased axial and circumferential compliance and flexibility and greater tissue ingrowth through the use of multiaxial fibril direction in a non-continuous outer ePTFE tubular body.
It is yet another advantage of the present invention to provide an improved method of forming such composites using preassembled graft/stent strips.
The present invention provides a composite intraluminal prosthesis for implantation which may have a substantially continuous ePTFE tubular inner body in combination with a non-continuous outer ePTFE tubular body formed by tubularly assembled polytetrafluoroethylene strips, or components. A circumferentially distensible support structure is interposed between the two PTFE layers. The components or strips comprising the outer tubular body possess a longitudinal length and a width, with said longitudinal length being greater than said widths; the non-continuous, tubular assembled strips providing axial and circumferential compliance to said prosthesis.
A method of forming an intraluminal prosthesis stent/graft with axial and circumferential compliance is provided by combining a non-continuous PTFE tubular outer body over a substantially continuous PTFE tubular inner body, said outer body and inner body supporting a stent thereinbetween. Use of a braided or woven PTFE in at least the outer layer enhances the axial and circumferential compliance, and provides puncture sealing properties to prosthesis of the present invention.